A single side-band (SSB) communication system requires accurate balancing of amplitude, offset, and phase between the in-phase (I) and quadrature (Q) channels for the best image rejection. A method widely used in the transmit channel is to use analog to digital (A/D) converters to accurately measure the amplitude and phase of the I and Q channels right before the single side-band mixer, and use that information in a digital signal processing (DSP) unit in a feedback configuration to adjust the gain, offset, and phase mismatch of the two channels, as illustrated in FIG. 1.
FIG. 1 is a schematic diagram of a conventional approach for I/Q channel balancing. The system 100 shown in FIG. 1 includes two analog-to-digital (A/D) converters 101, 102 coupled to a digital signal processing (DSP) unit 103. The A/D converters 101, 102 are able to sample the signal bandwidth of the I and Q channels 111, 112, respectively, but the information bandwidth extracted (signal amplitude, offset, and phase) in the DSP unit 103 is much lower than the signal bandwidth. Therefore, the A/D noise performance of the system 100 can be low to moderate due to averaging and bandwidth reduction in the DSP unit 103.
However, systematic mismatches between the I and Q A/D converters 101, 102, translate directly into mismatches of the I and Q amplitude, offset, and phase information, leading to reduced image frequency cancellation and local oscillator feedthrough. The A/D converters for such applications need very high matching performance, usually much higher than their noise performance.
One proposed system architecture designed to avoid the need for matched A/D converters is shown in FIG. 2. FIG. 2 is a schematic diagram of a conventional approach for I/Q channel balancing using a single A/D converter. In FIG. 2, a system 200 uses a single A/D converter 201 coupled to a DSP unit 203 and being clocked at twice the normal sampling rate. The system 200 multiplexes the I and Q channel signals 211 and 212 at its inputs. The perceived drawbacks of this proposed technique relate to the fact that the A/D converter 201 has to work at twice the previous clock rate, which may or may not be available in the system 200, and the fact that, due to time interleaving, the I and Q channel sampling does not occur at the same time, but there is a ½ clock time difference between the I and Q channel sampling. This time difference requires special processing in the DSP unit 203 and can have various unintended effects on the signal amplitude and phase estimation based on I and Q signal bandwidth compared to the A/D converter sampling rate.
It would be advantageous, therefore, to provide a method and system without the aforementioned drawbacks.